1. Field of the Invention
The invention relates to a method for regenerating alkaline solutions for pickling aluminum to recover aluminum hydroxide as well as to an apparatus for carrying out the method.
When aluminum surfaces are refined (e.g. anodic oxidation or plating), it is generally necessary to first subject these surfaces to a pickling or caustic treatment, where both acidic and basic pickling solutions can be employed. For alkaline pickling solutions, the aluminum is submersed into a heated bath, for example containing soda lye and is left there for a certain time. The pickling process consists of partially dissolving the metal surface by the lye, where small amounts of aluminum go into solution with the formation of hydrogen. The following total reaction takes place EQU Al+NaOH+H.sub.2 O.fwdarw.NaAlO.sub.2 +3/2 H.sub.2 ( 1)
Since a certain amount of aluminum dissolves, the concentration of aluminate increases continuously in the bath during operation. Disposal of pickling solutions containing larger amounts of aluminate represents both an ecological and also an economic problem. Up until now, these solutions were neutralized using large amounts of acid (which in some cases are available from eloxal baths). More recently however several attempts have been made to recover the aluminum contained in the pickling solution, where one precipitates it as aluminum hydroxide and reuses it as raw material. This saves the purchase costs of aluminum hydroxide recovered in crystalline form and at the same time avoids greater environmental problems, for example resulting from the disposal of the normally amorphous aluminum hydroxide sludge. Furthermore, when precipitating the aluminum hydroxide in stoichiometric ratio, the base used in the pickling bath is recovered: EQU NaAlO.sub.2 +2 H.sub.2 O.fwdarw.Al(OH).sub.3 +NaOH (2)
The reuse of the base in the pickling bath, set free during the precipitation process, results in a further cost advantage and a further reduction of environmental load, because a less highly alkaline waste water arises, which must be neutralized and purified, whereby a salt is formed.
A method is therefore desirable by which the sodium aluminate is continuously converted by precipitation to aluminum hydroxide and the resulting base is continuously reused in the pickling bath by means of recycling. Such a method would only consume water altogether and according to the following total equation, would convert aluminum into aluminum hydroxide while releasing hydrogen: EQU Al+3H.sub.2 O.fwdarw.Al(OH).sub.3 +3/2 H.sub.2 ( 3)
2. Description of the Related Art
Methods for recovering aluminum from alkaline pickling solutions in the form of aluminum hydroxide are known.
The German Patent DE-PS 43 977 discloses a method for treatment of hydrated alumina and alkali aluminate, by which aluminum hydroxide is precipitated by inoculation from a substantially pure alkali aluminate solution. This method was subject to later improvements, where however the improvements also used substantially pure aluminate solutions.
Modern aluminum pickling processes however do not work with pure alkali solutions, but have additives in the pickling solution which give the pickled aluminum surface a particular or desired optical appearance. Such additives are for example nitrate and nitrite. Today's pickling solutions also contain complexing agents mostly in the form of sorbitol, sometimes also gluconate. The additives provide that aluminum in the pickling bath is maintained as far as possible complexed in solution and does not prematurely precipitate as aluminum hydroxide. Such pickling solutions containing the above mentioned additives apart from aluminate are not suited for treatment with the above mentioned method of DE-PS 43 977 or the improvements deriving from this method.
EP-A1 157 190 discloses a method for precipitating aluminum hydroxide from pickling baths, which apart from aluminate also contain additives in the form of gluconate and carbonate. The precipitation of aluminum hydroxide in this method however takes place directly in the pickling bath. Such a procedure has drawbacks for various reasons. First of all, it cannot be prevented that the precipitated aluminum hydroxide does not deposit on the aluminum pieces to be pickled, which leads to undesired spot formation on the aluminum surface. In addition, it is necessary to filter out the precipitated aluminum hydroxide either continuously or batch-wise from the pickling solution.
The U.S. patent, U.S. Pat. No. 4,826,605 discloses the precipitation of aluminum hydroxide from pickling solution by the addition of acids and subsequent oxidation with ozone.
The object of the invention is to provide an improved method of the above mentioned type, by which the precipitation of aluminum hydroxide does not take place in the pickling bath and which enables inexpensive recovery of the aluminum hydroxide and also the base arising during precipitation by simple technical means.